1. Field of the Invention
This invention relates generally to power amplifier control. More particularly, the invention relates to an amplitude calibration element for enhanced data rates for a GSM evolution (EDGE) polar loop transmitter.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, portable communication devices are becoming more and more widespread. A portable communication device includes one or more power amplifiers for amplifying the power of the signal to be transmitted from the portable communication device.
With the decreasing size of portable communication devices, power efficiency is one of the most important design criteria. Reducing power consumption prolongs power source life and extends stand-by and talk time of the portable communication device. In a portable communication device that uses a non-constant amplitude output (i.e., one that modulates and amplifies both a phase component and an amplitude component), a linear power amplifier is typically used.
Unfortunately, a linear power amplifier has lower power efficiency than a nonlinear power amplifier and therefore increases the overall power consumption of the portable communication device. However, a non linear power amplifier can be used by employing separate feedback control of the amplitude modulation (AM) and phase modulation (PM) portions of the output signal. Using separate AM and PM feedback loops significantly reduces the problems with AM-to-PM conversion and AM-to-AM conversion when using a non-linear power amplifier. Such an architecture is referred to as a “polar loop” transmitter and uses separate feedback paths to control the amplitude and phase modulation. Further, the polar loop architecture allows for a large dynamic output power control range, which is desirable for the transmitter to operate in the Global System for Mobile Communication (GSM) standard.
One of the requirements for introducing the amplitude modulation into a power control feedback loop of a non-linear power amplifier is accurately detecting and calibrating the AM portion of the transmit signal so that it properly modulates and controls the power of the output signal. In the past, a diode detector has been used to detect and calibrate the AM portion of the signal. Unfortunately, a diode detector suffers from inaccuracy from temperature and manufacturing process variations, which must be compensated.
Therefore, it would be desirable to accurately calibrate the AM portion of the transmit signal without the need to compensate for variations in a diode detector.